Material to be welded for butt welding, methods of cutting as well as welding the same, and a wire

ABSTRACT

To provide a material to be welded for butt welding which can reduce contamination of particles into a welded portion after the welding is executed and which can construct piping or a device each enabling realization with high repeatability a ultra-clean atmosphere in which the density of impurity such as moisture or the like is suppressed, and a method of cutting the same. Also to provide a wire used for wire cut electric discharge machine which can prevent reduction of chromium composition in a place adjacent to the welded bead section after the welding is executed. (1) A face to be butted of the material to be welded is cut by a wire cut electric discharge machine. It should be noted that it is preferable that the surface of the wire used for the wire cut electric discharge machine is stainless or chromium and that stainless steel or chromium having a thickness of not less than 0.1 μm is deposited on the cut face. (2) As for the method of cutting, at least a face to be butted is cut by the wire cut electric discharge machine. (3) At least the surface of the wire is made of stainless steel or chromium.

FIELD OF THE INVENTION

The present invention relates to a material to be welded for buttwelding, a method of cutting the same, a method of welding the same, awire used for electric discharge machining, a fluid supply pipingsystem, and a process device.

TECHNOLOGICAL BACKGROUND

(A) In association with the tendency for higher degree of integrationand higher performance in the field of semiconductor devices, there aredemands for a manufacturing device which can satisfy the needs, therehave been made intensive efforts for creating a higher degree of vacuumand cleaner atmosphere for forming a film.

In order to create an ultra-high degree of vacuum and ultra-cleanatmosphere for forming a film, it is required to completely suppress outgas discharged from the manufacturing device (e.g., a process device forforming films or the like) as well as from the internal surface of afluid supply piping system for gas or the like.

With years of research and development activities by the presentinventor, now it is possible to form an oxide passivated-film on asurface of the device or the like containing therein chromium oxide as amain element which is excellent in corrosion resistance and emits onlyan extremely small amount of out gas therefrom. As a result, the presentinventor has succeeded in creating a supply gas as well as an atmospherefor forming a film in which out gas discharged from inside of the deviceis suppressed to a rate so that the gas is hardly detected by any ofcurrent measuring devices. Namely, it is possible to suppress a densityof impurities (moisture, hydrocarbons or the like) in the supply gas aswell as in the atmosphere for forming a film to not more than the orderof ppb and further to the order of ppt.

However, in accordance with the tendency that devices becomeincreasingly larger in size and more complicated, there have been moreand more intense needs for constructing a fluid piping for gas,chemicals or the like as well as a manufacturing device for asemiconductor or the like with materials to be welded each having anoxide passivated-film with chromium oxide as a main element thereofformed thereon made by cutting a long-sized member having an oxidepassivated-film with chromium oxide as a main element thereof formedthereon to a specified length, and by connecting the materials to bewelded to each other by welding.

It should be noted that conventionally the member is cut with a manualpipe cutter. Then, after the material is cut, the cut edge faces arefurther smoothed with bite by a specific edge-face processing device(produced by Tritool) to remote burrs due to the cutting, and also theroughness of the surface is made to be fine.

The cutting and the smoothing are executed while inert gas such as N₂gas or the like as purge gas is flown onto the surfaces being cut toprevent deposition of particles from outside as well as of chipsgenerated when the material is cut onto the surfaces of the material tobe welded. Especially, in a case where the material to be welded istubular, particles from outside as well as chips generated when thematerial or the like is cut go into inside of the pipe and are depositedon the internal surface of the pipe, so that it is especially requiredthat the cutting is executed while purge gas is flown inside the pipe.

(B) On the other hand, even if careful attention is paid to the mattergenerated when the material is cut, the oxide passivated-film iseliminated when the materials are welded, so that the surface of awelded bead section is not covered with the oxide passivated-film. Forthis reason, gas is easily deposited on and removed from the welded beadsection, which causes the supply gas to be contaminated. Further, in notonly the bead section but also in both sides of the welded bead section,chromium is volatilized, whereby chromium composition therein abruptlydecreases, so that the corrosion resistance is reduced, which is thefact found by the present inventor. FIG. 1 shows the state as describedabove, and in the figure the state is indicated by the solid line B.

Namely, as indicated by the solid line B shown in FIG. 1, the weldedbead section is set to 0, and when chrome composition is measured inboth sides of the section, it can be determined that the density ofchromium therein is abruptly decreased.

For this reason, when there are may joints to be welded, a quantity ofout gas discharged from the joints increases to an unignorable degree,whereby the atmosphere is contaminated, and realization of a ultra-highvacuum and ultra-clean atmosphere is prevented. Also in a device and apiping system each using corrosive gas or the like, a welded section anda place adjacent to the welded section are corroded, which also causesthe atmosphere to be contaminated.

For this reason, the inventor has proposed a technology different fromthe present invention, in which an oxide passivated-film with chromiumoxide as a main element thereof is formed also on the welded section bysubjecting the cut surface to chromium plating and welding, and thenexecuting processing for formation of a passivated film while gas isflown thereonto (Japanese Patent Application No. 5438/1993).

In this technology, the plated chromium has a roll of guarding chromiumfrom being volatilized, namely this plated chromium is oxidized byoxidizing gas due to heat generated when welding is executed, so thatthe chrome composition in the welded bead section as well as in a heataffected zone is prevented from decreasing, and an oxide passivated-filmwith chromium oxide as a main element thereof is also formed on thesezones. For this reason, with this technology, it is possible to suppressthe density of impurities in supply gas or an atmosphere for processingto not more than the order of ppb and further to the order of ppt evenin the fluid supply piping having a welded section as well as in themanufacturing device for a semiconductor or the like.

(C) However, the present inventor has found that the problems as followsoccur even in the technology described above.

(1) There is sometimes a case where an abnormally large number ofparticles are generated when welding is executed. Namely, as shown inFIG. 5, the problem described above has been found as a result ofwelding in a butt joint while back shield gas (B.S.G.) is being flowninside the tubular material to be welded (a sample) and countinggenerated particles with a particle counter in the downstream from theB.S.G.

The generated particles are deposited on the surface of the material tobe welded, and the material is used for construction of a process deviceor the like as it is, then, the deposited particles on the surface isremoved therefrom, for instance, when the gas is flown during formationof a film, and is brought into the atmosphere for forming the film.

(2) Also, particles go into the melting section, which causes thestrength of the welded section to be reduced.

(3) It is sometimes impossible to suppress the density of impuritiessuch as moisture or the like to the order of ppt. Namely, so far asrealization of a ultra-clean atmosphere is concerned, the repeatabilityof this technology is not always good.

It is an object of the present invention to provide a material to bewelded for butt welding which can reduce a quantity of particles goinginto a welded section after the welding is executed, and also to providea method of cutting the same.

It is another object of the present invention to provide a material tobe welded for butt welding which can construct a piping or a device eachbeing capable of realizing with high-repeatability or ultra-cleanatmosphere in which the density of impurities such as moisture or thelike can be suppressed, and a method of cutting the same.

It is still another object of the present invention to provide amaterial to be welded for butt welding excellent in strength in thewelded bead section and a method of cutting the same.

It is still further another object of the present invention to provide awire used for wire cut electric discharge machining which can preventreduction of chrome composition in a place adjacent to the welded beadsection after the welding is executed.

DISCLOSURE OF THE INVENTION

In the material to be welded for butt welding according to the presentinvention, a surface to be butted is cut by the wire cut electricdischarge machine.

The method of cutting the material to be welded for butt weldingaccording to the present invention is characterized in that at least asurface to be butted is cut by the wire cut electric discharge machine.

The wire used for wire cut electric discharge machining according to thepresent invention is characterized in that at least the surface thereofis made of stainless steel or chromium.

FUNCTION

Description is made hereinafter for effects of the present inventionwith reference to some knowledge obtained through the process to thepresent invention.

(A) The present inventor performed various types of experiment onrealization of the ultra-clean atmosphere described above to search thecause whey the repeatability of the atmosphere was insufficient.

One phenomenon was found in the process.

Description is made for the process. At first, an experiment wasperformed with a hollow pipe. A long-sized pipe with chromium oxide as amain element provided on the internal surface thereof was cut by thecutting method based on the conventional technology as described above,the edge surfaces were smoothed, then the materials each to be weldedwere butted to each other, and back shield gas (B.S.G.) consisting of amixed gas of hydrogen and argon was flown inside the pipe as shown inFIG. 5. The B.S.G. was flown so that the internal pressure of the B.S.G.inside the pipe would be 100 mmAg, and as a result of measuring theinternal pressure thereof after a certain period of time passed, asindicated by the line B shown in FIG. 3, the initial pressure indicated92 mmAg, however, the pressure dropped to 85 mmAg at the morment atwhich the welding was started, and exceeded 100 mmAg after a while.

The present inventor examined the reason why the internal pressure ofthe B.S.G. dropped, and guesses that the phenomenon had occurred due tothe reasons as follows. One of the reasons will be that, even if edgesurfaces are butted, as shown in FIG. 4(a), when the materials arebutted before welding, some force may be applied to the section to bewelded when the welding is started, which may cause a position oppositeto a starting position to be opened as shown in FIG. 4(b), and for thisreason, the internal pressure may drop. When the position opposite sideto the starting position for welding is also welded, inside of the pipeis shielded by welding, which may cause the internal pressure to go up,and this assumption may be correct because it matched with thephenomenon indicated by the line B shown in FIG. 3.

Then, it is conceivable that may particles may be generated in themanner to be welded based on the conventional technology because theparticles may go into inside of the pipe when this opening is made.

If the assumption described above is correct, the opening has to beprevented, and for this reason the inventor pursued for the means forsolving the problem. Because it was entirely unknown what was the causefor the phenomenon of opening, the inventor performed experiments undervarious conditions for welding as well as for clamping a material to bewelded or the like, but any significant effect over the phenomenon couldnot be found. Then, considering that the phenomenon might have somethingto do with a cut surface, the inventor performed on an experiment, atfirst, with surface roughness of an edge face changed on a trial basis.Namely, after the material was cut by means of the method based on theconventional technology, the cut surface of the material was smoothedwith bite to prepared samples having three types of surface roughnessrespectively such as (a) R_(a) =1500 nm, (b) R_(a) =650 nm, (c) R_(a)=500 nm, and the surfaces were butted to each other for being welded,whereby an experiment was performed, as a result, any significantdifference among the three types thereof was not recognized. Namely, itwas found that the fact that the surface roughness (R_(a)) was finecould not always prevent the generation of opening (the phenomenon ischecked by dropping of the internal pressure). Eventually, it was foundthat the surface roughness (R_(a)) was not always a dominant element forthe phenomenon.

Then, as a result of experiments performed by the present inventor tocheck various types of cutting method, it was found that a drop of theinternal pressure in the B.S.C. hardly occurred in the initial stage ofthe welding process in a case where the material was cut by the wire cutelectric discharge machining. Not only was it found, but also it wasfound that the internal pressure achieved 100 mmAg in the butted stateof the members. The state is indicated by the line A shown in FIG. 3.

It is conceivable that this state is generated because the edge facesare completely shielded and absorption force between the faces issufficiently strong.

Namely, in a case where the faces of the material cut by the wire cutelectric discharge machining are butted to each other, the butted facesare not opened even in any portion other than the starting position forwelding at the moment when the welding is started, and the entiresurfaces are shielded. For this reason, deposition of particles onto thesurface thereof can be prevented.

Also, entry of particles into the bead section can be prevented, whichmakes it possible to obtain high strength in the butted joint.

(B) On the other hand, it was found that the bead section was not flatwith pits and projections occurred thereon when the opening occurredtherein.

Then, the pits and projections found thereof each became a dead spacefor a fluid, which prevented the fluid from flowing smoothly. Thisphenomenon causes contamination of the supply fluid as a whole. It canbe considered that this mechanism will be obstacle to realize aultra-clean atmosphere for forming a film with high repeatability.

On the contrary, in a case where a material is cut with the wire by wirecut electric discharge machining, generation of the opening can beprevented, and as a result, a flat bead section with few pits andprojections thereon can be obtained, which makes it possible toeliminated a dead space thereon. Consequently, it is possible to supplya ultra-clean fluid as well as to realize a ultra-clean atmosphere forforming a film each free from any contamination.

(C) Also it was found that the cut faces of the material cut by wire cutelectric discharge machining are excellent in the characteristicsenabling deposition of chromium on the surface thereof as compared tothat on the face cut by the method based on the conventional technology.

Namely, it is found that chromium is not always iniformly deposited onthe face cut by the method based on the conventional technology. Thematerial was cut by the method based on the conventional technology andthe faces were welded after being subjected to chromium plating. Thenthe inventor developed, separately from the present invention, thetechnology in which an oxide passivated-film with chromium oxide as amain element thereof was formed on the welded bead section bypassivating the faces while oxidizing gas was flown onto the face andalso reduction of chrome composition in the heat affected zone wasprevented, however, the face that the density of impurities in the facecan not sometimes be reduced to the order of ppt even with thistechnology is as described in the column of related technology. Theinventor has made it clear that the reason described above is derivedfrom the fact that chromium is not always uniformly deposited on theface cut by the conventional type of cutting method.

However, the material to be welded having a face cut with the wire bywire cut electric discharge machining has such an effect that thematerial can have chromium uniformly deposited on the cut face. For thisreason, in a case where welding is executed while oxidizing gas is flownonto the face, it is possible to form an oxide passivated-film withchromium oxide on a welded bead section, consequently, a ultra-cleanatmosphere can be realized with high repeatability.

(D) As an edge face cut with the wire is excellent in smoothness andstraightness, it is extremely easy to butt materials to be welded toeach other (in a case of pipes, central axes are easily butted), so thatit is also possible to achieve other effect that a period of timerequired for a series of processes from cutting to welding can bereduced to about 1/3 as compared to that based on the conventionaltechnology.

EMBODIMENTS EXAMPLES

It is preferable that at least the surface of a wire used for a wire cutelectric discharge machine is made of stainless steel or chromium. Thestainless steel containing chromium therein by not less than 20percentage by weight is preferable, and SUS316L is one of the preferableexample.

The inventor has found that, when the surface of the wire is made ofstainless steel or chromium, some chromium is deposited on the cut edgeface of the material to be welded after the cutting is executed. Inaddition, it is found that a rate of the deposition of chromium thereonis not less than 0.1 μm which is sufficient for forming an oxidepassivated-film with chromium oxide as a main element thereof on thebead section as well as for preventing reduction of chrome compositionin the heat affected zone adjacent to the bead section. This fact is areason whey at least the surface of the wire used for the wire cutelectric discharge machine should preferably be made of stainless steelor chromium.

As described above, the fact that it is possible to form an oxidepassivated-film with chromium oxide as a main element thereof on thebead section as well as to prevent reduction of chrome composition inthe heat affected zone adjacent to the bead section if at least thesurface of the wire used for the wire cut electric discharge machine ismade of stainless steel or chrome, shows that it is possible to obtainsuch a large effect that disadvantageous extra processes such as platingor the like can be eliminated.

It should be noted that formation of an oxide passivated-film withchromium oxide as a main element thereof on the bead section may beexecuted by subjecting the faces to passivating while oxidizing gas isflown under a specified temperature for heating after the welding isexecuted, or may be executed by flowing chromium oxide as B.S.G. ontothe faces when the welding is executed.

The present invention is more effective in a case where the material tobe welded is tubular, and further more effective in a case where theouter diameter thereof is not mot more than 1/4 inch (6.35 mm).

In a case where the material to be welded is a pipe, alignment of twopipes is difficult and he opening can easily occur because an area inwhich the edge faces are contacted with each other obtained when thematerials to be welded are butted to each other is smaller than that ina case where the material is a solid. However, in the present invention,the alignment of two pipes can easily be made and generation of theopening can sufficiently be prevented even if the outer diameter of thepipe is not more than 1/4 inch (6.35 mm) and the wall thicknessthereofis 1 mm.

As a welding means for the present invention, for instance, a means withelectric discharge or with a laser is used. As a means with electricdischarge, tungsten inert gas welding and arc gas welding or the likecan be taken up as the example.

The process device according to the present invention indicates amanufacturing device for a semiconductor, a manufacturing device for asuper-conductive thin film, a manufacturing device for a magnetic thinfilm, a manufacturing device for a metallic thin film, and amanufacturing device for a derivative thin film or the like, and morespecifically, a spattering device, a vacuum depositing device, and adevice for forming a film as well as a processing device using CVD,PCVD, MOCVD, MBE, dry etching, ion implantation, a diffusion/oxidefurnace or the like, and also an evaluating device using such as Augerelectron spectrum, XPS, SIMS, RHEED, and TREX or the like. A pipingsystem for supplying gas to any of the devices described above as wellas manufacturing and supplying device for ultra-pure water are alsoincluded in the process device according to the present invention.

As a material to be welded, any of ferric stainless steel, austenitestainless steel, two-phase stainless steel is used. Especially, SUS316Lis preferable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing Cr composition in the top layer section afterwelding is executed;

FIG. 2 is a graph showing composition in a direction of the thickness ofa welded bead section after the material to be welded cut by wire cutelectric discharge machining is welded;

FIG. 3 is a graph showing changes in pressure in welding back shieldgas;

FIG. 4 is a sectional side view of the material to be welded showingchanges of a butted face during the welding;

FIG. 5 is a conceptional view showing a method of welding; and

FIG. 6 is a graph showing composition in a direction of the thickness ofthe material to be welded cut by the cutting method based on theconventional technology after the welding is executed.

BEST MODE FOR CARRYING OUT THE PRESENT INVENTION

Next description is made for an embodiment of the present invention withreference to FIG. 1.

Embodiment 1

There was prepared a SUS316L pipe having a diameter of 1/4 inch in whicha chromium oxide passivated-film with chromium as a main element thereofwas formed on the internal surface of a long-sized tube.

The tube was cut in a specified length by wire cut electric dischargemachining in which a wire was previously chrome-plated to be tubularmaterials to be welded.

As a result of measuring a film thickness of chromium deposited on theedge surface of this tubular material to be welded after the welding wasexecuted by means of XPS (X-ray photoelectron Spectrum), it was foundthat the film thickness was about 0.1 μm.

Then, the cut tubular materials to be welded were butt-welded to eachother with the tungsten inert gas welding method. As for the weldingconditions, the tube was welded in a cycle at a welding speed of 1cm/sec and a width of the bead section was adjusted to 2 mm. Ar gascontaining therein H₂ gas was used as back shield gas during welding.FIG. 5 shows the experimental method. Internal pressure of the backshield gas (B.S.G.) was recorded in a recorder through a pressuresensor. The initial pressure before the welding was executed was set to100 mmAg. FIG. 3 shows a relation between internal pressure of the BSGand a time in a range from the time immediately before the welding wasexecuted until the time after the welding was executed. The obtainedsamples of the welded pipe were classified as follows. B: a sample(indicating B shown in FIG. 1) which was subjected to edge-faceprocessing with the edge-face processing device based on theconventional technology (produced by Tritool), and A: a sample(indicating A shown in FIG. 1) in which chromium was deposited on theedge face due to electric discharge. In the edge-face pressing methodindicated by (B), the internal pressure showed 29 mm Ag when thematerial was set. The pressure was found to instantaneously drop until85 mm Ag during electric discharge for welding, then, the pressureincreased more and more in accordance with rising temperature in thewelded section. On the other hand, in the method of (A), the pressureshowed 100 mmAg, at a point of time when the material was set, which wasthe same value as the initial one, and it was found that the pressurenever dropped even when the welding was started.

After the welding was finished, a rate of chrome composition in the mostouter surface was checked in a range from the upstream by 100 mm fromthe welded bead section to the downstream by 35 mm therefrom to the flowof the back shield gas. FIG. 1 shows the result obtained by thechecking. The X-axis in FIG. 1 indicates measuring points, and theY-axis indicates a ratio of a detected rate of chromium to an entiredetected rate of Fe, Cr, Ni, and Mn.

As clearly understood from FIG. 1, the reduction of the composition rateof Cr in the bead section of a sample with chromium deposited on theedge face thereof was suppressed by about 10% to the background level.Namely a composition rate of Cr in the place adjacent to the welded beadsection is substantially equivalent to that in the most outer surface ofthe base material.

The pipe was cut after the welding was finished and a distribution ofthe composition in a depth direction of the surface of the weldedsection was measured with the XPS.

FIG. 2 shows a result of the measurement. In FIG. 2, the Y-axisindicates a composition ratio of atoms in the distribution, and theX-axis indicates an etching time or the surface by spattering. Anetching time for one minute corresponds to a film thickness of about 10nm to be etched.

As clearly understood from FIG. 2, in a case where a material to bewelded with chromium deposited on the edge face of the welded sectionthereof is welded, it is found that an oxide passivated-film containingtherein a large amount of chromium is formed on the welded section.

As a result of measuring a rate of particles by a particle counter, acase of the sample (B) had a rate of particles much less as compared tothat of (A).

Then, HCl gas containing therein moisture of 300 ppm was filled in thewelded pipe with 5 Kg/cm² of pressure and left as it was for 24 hours.Then, the pipe was cut to observe the internal surface thereof, and as aresult, corrosion was recognized on the surface of the welded section inthe sample (B), while corrosion was not recognized at all on the sampleof (A) like that on the non-welded section, so that it is found that anoxide passivated-film excellent in corrosion resistance is formed on thesurface with the welding method according to the present invention.

INDUSTRIAL APPLICABILITY

As described above, if a material is cut by electric discharge machinewith a wire subjected to chromium plating according to the presentinvention when the material to be welded is to be cut, it is possible todeposit chromium on edge faces of the material simultaneously when thematerial is cut.

When butt welding is executed to the materials as described above, awelded bead section with a thick chromium or a thick chromium oxide filmcan be subjected to passivation, which makes it possible to provide anultra-high-clean process device, a supply piping system for ultra-highpurity gas, and a manufacturing device for ultra-pure water.

Another big advantage in the cutting by this wire cut electric dischargemachining is to largely reduce a period of time required for the cuttingas well as edge-face processing. A series of processed based on theconventional technology such as cutting, edge-face processing, removinginternal burrs as well as external burrs can be executed by one process.Accordingly, the time reduction by around 1/3 with the present inventioncan be expected as compared to that with the conventional type ofmethod.

I claim:
 1. A material to be welded for butt welding, wherein at least abutted surface thereof is cut by a wire of a wire cut electric dischargemachine; andat least a surface of said wire being one of stainless steeland chromium.
 2. The material to be welded for butt welding according toclaim 1 wherein a chromium oxide passivated-film with said chromium as amain element thereof is formed on the surface of said material to bewelded before the material is cut.
 3. The material to be welded for buttwelding to claim 1 wherein said stainless steel or said chromium eachwith a thickness of not less than 0.1 um is deposited on said buttedsurface as said butted is surface cut by the wire.
 4. The material to bewelded for butt welding according to claim 1 wherein said material to bewelded is tubular.
 5. The material to be welded for butt weldingaccording to claim 4 wherein an outer diameter of said tubular materialto be welded is not more than 6.35 mm.
 6. A method of cutting a materialto be welded for butt welding; wherein at least a butted surface is cutby a wire of a wire cut electric discharge machine wherein at least asurface of the wire being one of stainless steel and chromium.
 7. Themethod of cutting a material to be welded for butt welding according toclaim 6 wherein said material to be welded is tubular.
 8. The method ofcutting a material to be welded for butt welding according to claim 7wherein an outer diameter of said tubular material to be welded is notmore than 6.35 mm.
 9. A method for butt welding wherein at least abutted surface thereof is cut by a wire electric discharge machine usinga wire having at least a surface of the wire being one of stainlesssteel and chromium; and the cut surfaces are butted to each other to bewelded.
 10. The method for butt welding according to claim 9; whereinthe welding is executed while back shield gas is flown in a placeadjacent to the welded section.
 11. The method for butt weldingaccording to claim 9 wherein said back shield gas is oxidizing gas orgas containing therein oxidizing gas, and an oxide passivated-film withchrome as a main element thereof is formed on a welded bead sectionsimultaneously when the welding is executed.
 12. The method for buttwelding according to claim 11 wherein said material to be welded istubular.
 13. The method for butt welding according to claim 12 whereinan outer diameter of said tubular material to be welded is not more than6.5 mm.
 14. A wire used for wire cut electric discharge machiningwherein at least the surface of the wire is made of stainless steel orchromium.
 15. A wire used for the wire cut electric discharge machiningwherein a stainless steel layer or a chromium layer is formed on asurface of a base of said wire made of elements other than stainlesssteel or chromium.
 16. A fluid supply piping system constructed bycutting at least a surface to be butted of a material to be welded witha wire of a wire cut electric discharge machine wherein at least asurface of the wire being one of stainless steel and chromium; andwelding said cut surfaces thereof in a butted state.
 17. A processdevice constructed by cutting at least the surface to be butted of amaterial to be welded with a wire cut electric discharge machine using awire having at least a wire surface being one of stainless steel orchromium and welding said cut surfaces thereof in a butted state.